1. Field of the Invention
The present invention relates to a magnetic bubble memory, and more particularly to an improvement in the performance of a series resonant drive circuit for generating a rotating magnetic field in a magnetic bubble memory device.
2. Description of the Prior Art
As is well known in the art, a magnetic bubble memory device usually comprises a magnetic bubble memory chip constructed to move magnetic bubbles in accordance with a rotating magnetic field, a magnetic bias field device for maintaining the magnetic bubbles at a constant size and a rotating magnetic field generating device for generating the rotating magnetic field. Of those, the rotating magnetic field generating device generates the rotating magnetic field in a plane of a magnetic bubble chip 3 which is, as shown in FIG. 1, disposed within X- and Y-coils 1 and 2 arranged orthogonally to each other. The rotating magnetic field generating device is also called a magnetic bubble driver, and various rotating magnetic field generators have been proposed so far. As one example thereof, a series resonant drive circuit for the magnetic bubble memory has been proposed by the present assignee. (See U.S. Pat. No. 3,763,478 dated Oct. 2, 1973 corresponding to Japanese Patent Publication No. 50-34381 entitled "Series Resonance Drive Circuit for Magnetic Bubble Memory".) In the proposed series resonant drive circuit, as shown in FIG. 2, a series resonance circuit is formed by connecting a resonance capacitor 4 in series with the X-coil 1 and another series resonance circuit is formed by connecting a resonance capacitor 5 in series with the Y-coil 2. These resonance circuits are connected to drive sources 6 and 7, respectively. The drive sources 6 and 7 comprise D.C. power sources 6-1 and 7-1, and switching devices 6-2, 6-3 and 7-2, 7-3, respectively. The switching devices 6-2, 6-3 and 7-2, 7-3 are turned on and off as shown in a time chart of FIG. 3. Through the switching operations of the switching devices 6-2, 6-3 and 7-2, 7-3 in the drive sources 6 and 7, sinusoidal wave currents I.sub.X and I.sub.Y having a phase difference of 90 degrees therebetween flow through the X-coil 1 and the Y-coil 2, respectively. The series resonant drive circuit for the magnetic bubble memory is characterized by a very small power dissipation as compared with the case where the coils are driven directly by A.C. power supplies. As a result, the drive source circuits can be simplified and the cost thereof can be reduced.
However, when a magnetic bubble memory was manufactured using such a series resonant drive circuit and the rotating magnetic field was generated to operate the magnetic bubble memory, the following problem was encountered. That is, the strength of the rotating magnetic field varies greatly with the direction thereof, and the rotating magnetic field applied to the magnetic bubble memory chip 3 varies widely with memory modules. In order to stably operate the magnetic bubble memory chip 3, it is necessary for the variation in the strength of the rotating magnetic field to be within a range of .+-.10 percent of a rated value. However, the strength of the rotating magnetic field in a magnetic bubble memory varies widely, that is, a -50% to +20% variation in the strength of the rotating magnetic field is observed, and therefore it is impossible to stably operate the magnetic bubble memory chip 3. Such a variation in the strength of the rotating field becomes large as the frequency of the rotating magnetic field is made higher.